1. Field of the Invention
The present invention relates to a resin molding method and a resin molding apparatus used for manufacturing a plastic-sealed semiconductor device.
2. Description of the Related Art
A transfer molding apparatus is heavily used for manufacturing a semiconductor device through plastic molding. In a plastic molding apparatus of this type, a resin tablet is supplied to a pot. A lead frame to be molded is clamped between an upper molding die and a lower molding die, and resin melted in the pot is extruded from the pot by means of a plunger. The thus-extruded resin is filled into a cavity and thermally set, thus sealing a semiconductor element.
However, under the foregoing method by which a semiconductor element is sealed with resin by feeding a resin tablet to the pot and by forcefully feeding the resin melted in the pot to the cavity, molded resin is susceptible to mixing of air, because the resin tablet itself is liable to contain gas, and gas is likely to be mixed into resin during the course of filling resin into the cavity. Molding resin under a great dwell pressure in an existing transfer molding apparatus is intended to reduce the volume of air mixed into resin to as small a volume as possible.
In order to apply a great dwell pressure to the resin molding apparatus, it has been necessary to robustly manufacture molding dies. Further, a hydraulic press is used for applying a great clamping force to the molding dies. In a case where a resin tablet made by compacting resin is used as is used for an existing resin molding apparatus, melted resin does not necessarily have sufficient flowing characteristics, resulting in a problem of so-called wire sweep or the cavity being not filled with resin (hereinafter often referred to as "a non-filling failure").
To solve the foregoing problems, there has already been proposed a method of molding resin by feeding to a pot liquid resin in place of a solid resin tablet (Japanese Patent Application Laid-open No. Hei-8-340015). In short, this method is one by which resin is molded by feeding liquid resin to a pot while the resin is sealed with a plastic film. Use of liquid resin contributes to an improvement in the filling characteristics of resin in the cavity and prevention of wire sweep, because liquid resin has the superior flowing characteristics. Therefore, the foregoing method has the advantage of being able to reliably mold resin by preventing air from being mixed into the resin.
The existing resin molding apparatus has a pot, a cull, runners, and gates as a resin flow channel connecting the pot to the cavity. Resin remaining in the resin flow channel such as the cull or the runners after setting of resin is undesired resin which does not contribute to the molding of resin. There sometimes arises undesired resin which is substantially equal in amount to the resin filled into the cavity. Thus, the foregoing method suffers a problem of resin being wasted.